Crash Course Astronomy

Host Phil Plait explains the nature and origins of the field of astronomy. Learn how ancient humans observed the sky and associated the patterns they found there with farming and the seasons on Earth. Early astronomers developed an understanding of our place in the Milky Way using math and science. Explore where our world exists within the universe, the origins of the field of astronomy, famous astronomers, and advancements made to the science that have widened our view and our knowledge of the universe.

Host Phil Plait invites you to head outside and take a look at all the incredible things you can see with your naked eye. Learn about constellations, and the visible stars and planets that can be found in the night sky. The brightness and color of stars, how they can be viewed, and the problem of light pollution is also discussed.

Take a look at the cyclical phenomena at work in the universe. Learn about the subtle changes in the sky that take place over time as the Earth circles the sun, following along the ecliptic. Learn how the tilt of the Earth's axis has a profound effect on the planet by creating the seasons. The Earth's axis is slowly moving as well, in a process called precession.

Learn why the moon has phases and what those phases are. Because the moon is a sphere and orbiting the earth every 29.5 days, the way we see it in the night sky changes with time. Learn what the phases of the moon are: why they occur, how we see them, and what they are called.

In this episode, Phil breaks down what happens during a solar and lunar eclipse and provides tips for safely viewing a solar eclipse. Learn how the Ancient Greeks calculated a close estimation of the earth and moon's actual size two thousand years before the invention of the telescope.

Host Phil Plait explains how telescopes work and offers up some astronomical shopping advice. In this episode, we learn how telescopes do two things: collect light, and increase our ability to resolve details. Learn the difference between refracting and reflecting telescopes, and how the discovery of the light spectrum influenced technology and increased our understanding of the universe.

Learn how gravity plays out across the universe. In this episode, the forces of gravity at home and in space are explored. Gravity accelerates things that have mass. Objects that move along a path controlled by gravity are said to be in orbit. We also learn the different types of orbits, and what an escape velocity is.

Explore the world of tides, and learn how they have shaped most objects in the universe. What is the relationship between tides and gravity? How do planets and their moons become tidally locked? What would happen if you were 300km tall? Find out in this episode.

Phil takes us for a closer look at the two-octillion-ton star that rules our solar system. We look at the sun's core, plasma, magnetic fields, sunspots, solar flares, coronal mass ejections, and what all of that means for our planet.

Learn all about planet Earth, from the composition of its internal structure and core, to its shifting continental plates, and placement in the solar system. We take a tour of Earth's inner and outer core, the origins of the planet's magnetic field, and the elements of our unique world that keep us safe and sustain life.

Take a tour of the moon, the closest astronomical object to Earth. Learn theories of the moon's formation, understand its surface features and how they formed, like the highlands and maria, and the moon's internal structures.

Venus is a gorgeous naked-eye planet, hanging like a diamond in the twilight, but its beauty is best looked at from afar. Even though Mercury is closer to the sun, Venus is the hottest planet in the solar system due to a runaway greenhouse effect. It has the most volcanic activity in the solar system. Learn more about this beautiful but inhospitable world.

The fourth planet from the sun and the outermost of the terrestrial planets, Mars has long been a popular spot for missions and imagination. Learn about the planet's topography, core, and features. We'll take a look back to Mars's past and make predictions for its future, including the possibilities for human life.

Jupiter is the biggest planet in our solar system. The gas giant is not a failed star, but a really successful planet! It has a dynamic atmosphere with belts and zones, as well as an enormous red spot that's actually a persistent hurricane.

Before moving on from Jupiter to Saturn, we're going to linger for a moment on Jupiter's moons. There are 67 known moons, and four huge ones that we want to explore in greater detail. Ganymede is the largest - larger, in fact, than any other moon in the solar system and the planet Mercury! Callisto, orbiting the farthest out, is smaller but quite similar to Ganymede in many ways. Io, meanwhile, is most noteworthy for its tremendous volcanic activity. There's also water on Ganymede and Europa!

We complete our planetary tour with a review of the ice giants Uranus and Neptune. Both have small rocky cores, thick mantles of ammonia, water, and methane, and atmospheres that make them look greenish and blue. Uranus has a truly weird rotation and relatively dull weather, while Neptune has clouds and storms whipped by tremendous winds.

Asteroids are chunks of rock or metal, or both, that were once part of a planet, but were destroyed after collisions. Most asteroids orbit the Sun between Mars and Jupiter, but some get near the Earth. The biggest asteroid, Ceres, is far smaller than the Moon but still big enough to be round and have undergone differentiation.